1. Field of the Invention
The present invention is related to the maintenance of communications networks. More particularly, the invention is directed to improved monitoring of transmissions errors in telecommunications networks.
2. Discussion of Related Art
It has been known to provide synchronous-transfer modes (STM) control of traffic using frames for such services as T-1, T-3 and others. More recently, hybrid ATM/TDM networks provide synchronous-transfer mode (STM) control of traffic from both the ATM and TDM payload sources. They permit a graceful, piecewise implementation of the megabits-per-second speeds of asynchronous transfer mode (ATM) packet-routing segments within existing voice, fax and data-communications networks having conventional digitally-switched call circuits (DSCs) that carry STM communications. STM communications use a signal-reference frame that provides dedicated time slots for the time-division multiplexing (TDM) of signals passing over the individual links in an STM network.
In theory, it should be possible to use the TDM trouble shooting criteria for monitoring the quality of hybrid ATM/TDM service and obtain the level of quality and efficiency achieved in purely TDM networks. However, field experiments that attempted real-time detection of transmission errors in call circuits combining ATM and TDM payloads resulted in imprecise descriptions of the troubles. The potential throughput efficiency advantages of the hybrid ATM/TDM systems cannot be realized so long as false-positive trouble flags mask the true nature and location of trouble spots in a network.
Even for purely TDM payloads, the conventional reporting of transmission errors as “errored seconds”, does not accurately reflect the condition of the data actually received. The errored seconds are measured using parity bit (PB) or cyclic redundancy check (CRC) analysis, but they are determined using a fixed frame clock. In this time-locked conventional error reporting scheme, out-of-frame format errors (OOF) preempt the results of parity and CRC tests, that is, every frame transmitted while an OOF condition exists is a continuation of the transmission error, regardless of the accuracy of the data within that frame. When both the PB and CRC errors and the OOF errors are reported together as a total length of time, the entire time elapsed before the expected frame format reappears with the correct intra-frame parity or redundancy, is reported in those “errored” or “OOF” seconds.
Conventional TDM service profiles reporting OOF “errored seconds” are compiled by terminal equipment such as the DDM-1000 or the DCS 3/1, or an interface unit (IU) that is an adjunct to such terminal equipment.
Over-reporting of transmission errors is particularly serious for the digital service providers (DSPs) who are subject to tariff regulations. Often times the detection of transmission errors fails to distinguish between the types of errors. This distinction could be important since a DSP must be prepared to defend its performance as a licensee by proving the quality of their service and the reliability of the service that they have provided to the public under their operating licenses.
Greater accuracy in quantifying data losses, and greater detail reporting the occurrence of events that disrupt TDM communications, is especially important for trouble analysis in long-distance circuits. The complex interactions that occur when the service provided to a customer involves multiple companies, during “bridge and roll” re-direction of calls, for example, are accompanied by an increase in the incidence of abnormal service interruptions of unknown origin.
In hybrid ATM/TDM networks voice traffic is, in part, carried on asynchronous packet-switched circuit. Terminal adapters (TA), operating in the ATM domain reformat the payload received from conventional TDM networks into packets. In the ATM domain, voice and data are entirely recoverable from TDM inputs having OOF errors of less than a frame. Unfortunately, using standard TDM error criteria that slight OOF timing error will be reported as a serious outage until framing is reset. OOF-based error reporting has been a blunt instrument for analyzing trouble in conventional TDM payloads. In the new hybrid ATM/TDM networks the instrument is even less useful as it swamps the TDM parity and redundancy test and other measures that reflect actual signal degradation at the interface between TDM and ATM systems.
The data rate at the payload inputs to ATM networks need not be fixed and the relative packet timing across an ATM call is highly variable. There are three principal sources of delay in ATM communications: queuing delays in the packet switches; digital voice compression coding/decoding delays; and voice-packet assembly delays. In particular, the quality of a voice signal transmitted across an ATM network is sensitive to round-trip delay and packet-order error, but not to “framing errors” or even individual dropped packets. Thus, the Terminal Adapters on the boundary between ATM and TDM networks that report OOF incidents as call outages do not accurately describe the frequency or extent of any impairment of the voice signal received by the ATM portion of the network, nor by the customers using the hybrid networks.
The present invention provides constant-frame error detection with synchronization criteria to provide a more precise signature of a circuit impairment.